1) Field of the Invention
The present invention relates to a network system which constitutes a virtual local area network (LAN), a transmission method and a computer program.
2) Description of the Related Art
As network systems, there is known a virtual LAN (VLAN) technique which virtually constitutes a LAN as if a plurality of different LANs exist at the same physical port. This technique is specified by IEEE802.1Q or the like and applied. According to this VLAN, tag identifiers (TPID) are provided in packets to be transmitted, respectively, thereby grouping terminal equipment or the like which are connected to LANs. For example, if a VLAN is constituted among a plurality of routers to each of which a plurality of terminals are connected, the terminals are grouped by identifying TPIDs in the packets which pass through the routers. Needless to say, if a VLAN is constituted among a plurality of switching hubs to each of which the terminals are connected, the terminals are grouped by identifying TPIDs, as well. Each router or switching hub exercises control so that data can be transmitted among the terminals which belong to the same group and that data cannot be transmitted on a data link layer among different groups. Therefore, from the terminal side, it appears that a plurality of different LANs exist. From the router or switching hub side, different VLANs can be multiplexed at the same physical port. It is, therefore, possible in the VLANs to decrease labor required to lay a network cable, labor required to switch over connection during regrouping or the like. Moreover, since data cannot be transmitted on the data link layer among different groups, it becomes advantageously easy to ensure security, manage terminal addresses by grouping LANs and the like.
However, bands which the users of VLAN thus multiplexed can utilize are preferentially allocated to a user who accesses the network precedently (precedence priority system). Therefore, if the user transfers data which has a wide transmission band, the data occupies all the bands of a physical link which constitutes the VLAN and the other users cannot hold communication.